The present invention relates generally to capacitors. More particularly, the present invention relates to thin film capacitors which are protected against electrostatic discharge.
Capacitors are known in the art. Traditionally, capacitors are constructed by placing a relatively thick layer of dielectric between two conducting plates. By way of example, older generation thin film capacitors have a layer of oxide or a combination of oxide and nitride about 2,000 angstrom thick as the insulating dielectric. Thick film capacitors would have a dielectric thickness of even ten times greater than this. Such thick film capacitors are usually relatively immune to electrostatic discharge (ESD) damage. This is because a capacitor's breakdown voltage is proportional to the thickness of its dielectric layer according to the formula: EQU BV=L.times.D
where L is the dielectric strength and D is the thickness of the insulating dielectric layer. It is not uncommon for thick film capacitors having a fairly thick dielectric layer to have an intrinsic breakdown voltages of hundreds of volts, and electrostatic discharge protection value of up to 1,000 volts.
However, the trend toward miniaturization requires that components, such as capacitors, on a circuit board be increasingly miniaturized. It is known that a capacitor's capacitance is proportional to the area of the plates (A) and the dielectric constant (K) of the insulating material, and inversely proportional to the distance between the plates (d) according to the formula: EQU C=K.times.(A/d).
Consequently, there are essentially three ways to increase a capacitor's capacitance: 1) find some dielectric material that provides more capacitance per unit area without degrading the breakdown characteristics, 2) increase the area of the plates, and 3) make the dielectric film thinner. If the same dielectric material is used, increasing the area of the plates is usually uneconomical when capacitors are fabricated as part of an integrated circuit on a silicon die since die space is very expensive. Reducing the thickness of the dielectric film has the advantage of both increasing the amount of capacitance and reducing the size of the capacitor. Known as thin film capacitors, these devices have the ability to be integrated in very close proximity with other devices, such as resistors.
However, as the thickness of the insulating dielectric layer is reduced, the resulting capacitor becomes more susceptible to electrostatic discharge (ESD) energy. This is because the breakdown voltage of a capacitor, as mentioned earlier, diminishes as the film of dielectric is thinned. If electrostatic energy gets discharged across an unprotected thin film capacitor, dielectric damage or outright destruction of the device can result.
To protect thin film capacitors against ESD-related damages, there is typically provided a protection mechanism that furnishes a parallel discharge path to protect the layer of insulating dielectric in the event the entire structure experiences ESD discharge. The protection device typically has a breakdown voltage which is suitably below that of the capacitor dielectric itself. ESD energy then gets dissipated in the parallel discharge path through the protection device before it can damage the insulating dielectric layer.
FIG. 1 shows, for illustration purposes, a prior art I.C. protection device as applied to a single capacitor structure 100 including capacitor 102, and protection device 104. Protection device 104 includes diodes 106 and 108 in series between ground and Vcc. When the potential at pin 110 is highly positive, e.g., due to the presence of electrostatic discharge energy, current is discharged through path 112. Alternatively when pin 110 is highly negative, current is discharged through path 114. In this manner, destructive current is discharged through protective device 104 before damage to the insulating dielectric layer within capacitor 102 could occur. However, the thin film ESD-protected capacitor structure 100 of FIG. 1 only looks like a capacitor when the applied voltage is between Vcc and ground. Further voltage swings forward bias the diodes and destroy the effectiveness of the capacitor. This represents a significant disadvantage. Further, an extra pin is required (for Vcc) in order to protect capacitor 102. For most applications, the extra pin requirement is highly undesirable, as standard capacitors do not come with power supply pins.
FIG. 2 shows, for illustration purposes, another prior art capacitor structure, including thin film capacitor 200 and protection device 202. Protection device 202 includes an n-channel transistor 204, which typically has a high threshold voltage, say 12 to 20 volts. When the potential of pin 110 is above the threshold voltage of n-channel transistor 204, it turns on transistor 204 to discharge current along path 206. When the potential at pin 110 is below ground, current is discharged through path 208 via a diode 210. However, this prior art ESD protection scheme of FIG. 2 is unipolar, i.e., it can only be used for voltages above ground, which is undesirable for many applications.
Further, the processes currently available for manufacturing these prior art protection devices in combination with thin films impose the requirement that connections to the capacitor be surface-oriented. This inserts unwanted resistance and inductance in series with the capacitor, which deteriorates performance at higher frequencies. It also puts significant non-linear capacitance into the circuit because of the effects of the protection device. This poor high frequency characteristic renders the prior art capacitor structure particularly unsuitable for use in modern high speed circuitries.
In view of the foregoing, what is desired is a thin film ESD-protected capacitor structure that does not have forward conduction in either direction, e.g., can be polarized either way within a useful voltage range, but still can offer good ESD protection. The improved thin film ESD-protected capacitor structure preferably maximizes the high frequency characteristics while lending itself to cost-effective semiconductor and thin film manufacturing techniques.